Rollup merge of #49968 - christianpoveda:stabilize_dyn, r=nikomatsakis

Stabilize dyn trait

This PR stabilizes RFC 2113. I followed the [stabilization guide](https://forge.rust-lang.org/stabilization-guide.html).

Related issue: https://github.com/rust-lang/rust/issues/49218

src/librustdoc/lib.rs

@@ -13,6 +13,8 @@
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/")]
 
+#![cfg_attr(stage0, feature(dyn_trait))]
+
 #![feature(ascii_ctype)]
 #![feature(rustc_private)]
 #![feature(box_patterns)]
@@ -23,7 +25,6 @@
 #![feature(test)]
 #![feature(vec_remove_item)]
 #![feature(entry_and_modify)]
-#![feature(dyn_trait)]
 
 extern crate arena;
 extern crate getopts;